filtering is the best option, set a bandpass then automate central freq. by following the vocal melody, then export as a wav for cutting up. It would wager will be hard to achieve satisfactory result(s).

if you have the instrumental it can be done by phase inversion, its not easy though... and theres so many factors that would make it impossible

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Just though id add my 2 cents, I also have seen it done with phase inversion but no hardware was required. The guy did it in audacity and all he needed was a full copy of the song plus a totally identical instrumental version of the song too.

As far as i know both versions have to be of exact equal quality but i am unsure of if it leaves the vocal natural or not as i cant remember now.

Dont hold me to this but i think it was a WARBEATS tutorial i watched, anyways good luck i just though the info might come in handy for someone especially as his method uses a free program and does not require any additional hardware.